Converters are usually used to convert electrical energy from one form to another. Converter topologies can for example be classified as boost, buck or buck-boost converters where all three topologies can be implemented as a DC to DC or an AC to DC converter. Unless otherwise mentioned, the terms boost converter, buck converter and buck-boost converter as used in this description shall include the DC/DC topologies as well as the AC/DC topologies.
AC/DC boost converters are often used for power factor correction (PFC) in the first stage of a multi-stage switch mode power converter. Such boost converters are usually designed to work either in discontinuous (DCM) or continuous conduction mode (CCM). Since the gain of the current control loop shifts considerably between those modes, a mixed conduction mode (MCM) leads to current steps at transitions which negatively affect the harmonic distortion of the input current. On the other side, mixed mode operation results in a higher efficiency with comparable magnetics sizes especially for medium and higher power levels.
Such an AC/DC boost converter is employed to transfer energy from an AC supply network to its output such that the current follows the input voltage. Either analogue or digital controller designs may be employed to control such converters. Known controller designs often include an inner current loop, an outer voltage loop and possibly also a multiplier with an input voltage feed forward to connect both loops.
The document “Digital Control of Boost PFC Converters Operating in both Continuous and Discontinuous Conduction Mode” (Gusseme et al.; 35th Annual IEEE Power Electronics Specialists Conference, Aachen 2004, p 2346-2352) deals with a digital control of boost PFC converters that can be operated in continuous conduction mode (CCM) as well in discontinuous conduction mode (DCM). In order to avoid input current distortion when switching between CCM and DCM a duty-ratio feed-forward is suggested where the optimal duty-ratio is calculated as a combination of the duty-ratios for both conduction modes and then added to the output of the controller. However, since the feed-forward signal is added only after the current compensator, the controller is not immune to noise wherefore it may not be used in commercial products.
The document “Digital control for improved efficiency and reduced harmonic distortion over wide load range in boost PFC rectifiers” (Chen et al.; 2009; Power Electronics, IEEE) discloses another controller for a boost PFC rectifier employing a predictive current control technique for CCM operation. It is suggested to modify this control technique for DCM operation by introducing a current correction factor. By adding an auxiliary inductor winding and a voltage comparator for detecting zero crossings of the inductor voltage a simple calculation of the correction factor is enabled. The suggested solution however results in a complex additional network increasing not only the volume but also the costs of such a controller.
The document “Adaptive tuning of switched-mode power supplies operating in discontinuous and continuous conduction modes” (Morroni et al.; 2009; Power Electronics, IEEE Transactions, p. 2603-2611) discloses an adaptive controller for SMPS, in particular for transitions between CCM and DCM operation. A Digital Stability Margin Monitor feeds a square signal into the closed loop between the compensator and the PWM where the frequency of this square signal is chosen such that the crossover frequency equals its frequency. The loop gain phase margin is measured and the adaptive controller determines the comparator coefficients such that these frequencies meet the desired values.
The known controller designs typically have a dissatisfying total harmonic distortion (THD) and/or power factor (PF), are prone to noise and/or are complex in design and expensive.